This book aims to tackle the United State's decision of entering a second war against Iraq in 2003. The decision is studied as a phenomenon resulting from a national and international bargain (two level game). In a first chapter the context as well as the actors are introduced. The emphasis is focused on the Bush System: a basis of governing introduced by the US President George W. Bush, that overruled the national and international practices of governing of his administrations. The window of opportunity represented by the September 11 events is also put forward in order to explain the conditions of the intervention. The second chapter deals with the study of the decision-making process through paradigms such as these of Robert Putnam and Graham Allison. Coming to the conclusion that these lenses of analysis are still perfectible in order to explain the remaining blurriness of the decision-making process, I share the results of my very own research in a last chapter. The third chapter hence focuses on the concept of collusion in the field of political science as a paradigm. It is introduced, conceptualized and applied to the subject of this thesis in order to enlighten it.